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Coloured versus Grey Undercoats – Trials with Interference Colours
- May 18, 2018 -


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This bonnet has first been painted in white (Uni), From below to above, the stripes have been painted with different interference colours. Depending on the viewing angle, the reflection colours or transmission colours can be identified over the white undercoat. Over a black undercoat only the reflection colours are visible. The corresponding transmission colours are absorbed by the black undercoat.



Modern car paint usually consists of a 2-layer structure: the base coat contains pigments for colours and effects, whilst the clear top coat protects the base coat from mechanical and chemical influences. In addition to coloured pigments, modern car paints contain aluminium or interference pigments which create different effects. 

Coloured pigments absorb part of the incoming light and scatter the rest in all directions. Aluminium pigments reflect the light and create high reflective effects similar to gloss. And since the middle of the eighties, interference pigments have been used in car paints. As the name implies, their colours are not created by absorption and reflection, but through the interaction of light waves. This principle is also found in nature, for example in beetles and butterfly wings. 

Interference pigments were first used alone; before they were mixed with coloured pigments to expand the range of colours and effects. Nowadays, in most cases, all three pigment types are used together to create the paint. For example, a fine aluminium pigment increases the opacity of the paint. The use of interference pigments also allows unusual colours and effects; however, this may reduce the hiding power. 

In many cases, a coloured or grey filler is applied when the hiding power is inadequate. The filler colour here should compensate for the transparency of the overlying base coat, i.e. the colour of the not-quite-opaque base coat is invigorated by the underlying filler. We can differentiate the individual components – paint layers and pigments – from one another as little with our eyes as with colour measurement instruments. The overall colour impression is what is always presented. The different aspects are to be illustrated in two different trial series. One will deal with an individual interference pigment and the other with a real series colour. 

Many interference pigments are transparent and show two colours or colour effects. They consist of a carrier material that is coated with a strongly diffractive layer of a metal oxide – for example titanium oxide. The surface of the pigment reflects part of the light falling on it and this also happens on the border layer with the carrier material. Both light parts interfere with each other and typical reflection colours are generated. These are particularly dependent on the layer thickness of the metal oxide and on the angle of the light. Similar processes take place on the rear side of the pigment, although the resulting transmission colour is complimentary to the reflection colour because of the missing phase shift. For example, a pearl green reflects strongly in the green spectral range. When examined, the corresponding complementary colour appears in the red spectral range.

The transparency of these types of inter ference pigments leads to a strong dependence of the overall colour impression on the undercoat colour. If a transparent interference pigment is used in a base coat system and this is applied to a black and a white base coat/filler, the results show two extremes: the black undercoat absorbs nearly all light rays that fall on it, and the white reflects nearly all. And the complementary transmission colour of the interference pigment is also reflected from this latter. This same effect presents itself in both visual colour matching and with instrumental measurement: the reflection colour can be identified close to the radiance. In the region between 20° and 30° from the gloss, there is a transition area in which the change to complementary transmission colour takes place. This change can be illustrated when the corresponding colour values are plotted against the measurement geometries. With a green interference pigment, the change between the green reflection colour and the red reflection colour becomes visible when colour value a* is selected (Figure 2). 

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The reflection over a black undercoat is lower than over the white. With a green interference pigment, the red transmission colour is reflected by the white undercoat. As the light is separated into reflections and transmissions, both parts together produce white again. While the chroma over the white undercoat first decreases with the increasing differential angle (aspecular) and then rises again after the transition area, it lowers continuously over the black undercoat. A similar result can also be seen for the brightness: it is higher over the white undercoat than over the black. This also first decreases with the increasing differential angle and then rises again. Over a black undercoat, it falls continuously (Figure 3). 

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The measurements have been carried out using the most up-to-date devices – X-Rite MA98 and BYK-mac I. It is important to note here that the measurement angles are not at equal distances. The difference between measurement positions increases in unequal steps; in this respect, the statements relate generally to these measurement geometries. Interference pigments can be measured in a limited way with both devices. The interference of a pigment or a paint is shown by changes in the incidence angle of the light. The BYK-mac device offers just one illumination geometry and the MA98 offers two. The interference can sometimes be shown by a trick; it must be noted, however, that the results depend strongly on the type of application (Figure 4). 


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The trials show the dependence of transparent pigments on the undercoat colour. White and black represent the two extremes. In between are graduations or grey or coloured undercoats. Both possibilities are applied in both automotive OEMs and repairs painting (refinishing). The search for better methods – whether with or without colour – often reaches philosophical dimensions, although the factual situation is clear: the overall colour impression determines the result, both in terms of visual assessment and also in instrumental measurement. And the overall colour impression consists of different components. Firstly, a large part of the light is reflected from the base coat. This reflection can consist of reflections from reflective aluminium pigments and reflections from and within interference pigments. Absorbing colour pigments spread the light – even the reflected light from the aforementioned pigment types – in all directions. The processes in the base coat are very complex, as here different types of pigment and their different optical properties come together. Depending on the transparency of the base coat, the remaining proportion of the light penetrates this base coat and escapes through the underside. This proportion of the light is also subject to complex optical processes in the base coat and this then hits the undercoat. 

The transparent base coat acts like a transparent colour film with white light shining on it. Part of the light is reflected, another part is absorbed and a further part leaves the film through the other side. This part then hits the undercoat. And depending on the colour, it receives different treatment here. A red undercoat appears black when it is irradiated with green light, and vice versa. In order to report the possible effects of a transparent base coat on a coloured or grey filler, a test series has been initiated with the standard colour Lucifer Red from Peugeot.

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With white light, the four increments of white to black are identifiable along with the colours. If the same sample panel is irradiated with red light, yellow and red change into white and/or light grey. Green and blue do not reflect any red light, so they appear black. A red base coat shows similar optical reactions over coloured undercoats.

Test set-up 

Materials were acquired from PPG Refinish and the standard colour Lucifer Red was mixed according to paint formulation. The filler colours were simulated with mixed paints: one green, one red and one red-violet mixed paint. All mixed paints were applied pneumatically to sample panels according to the manufacturer’s instructions. And all mixed paints were prepared with white mixed paint in the ratios 80:20, 60:40, 40:60 and 20:80 and correspondingly applied. In a second series, grey tones were created also in line with recipes stipulated by PPG Refinish as grey graduations from SG01 to SG06 (Spectral Grey). Then Lucifer Red was applied pneumatically as a base coat on all painted sample panels in two spray procedures. As a reference, a further panel was sprayed with the base coat until no transparency could be identified (SW Monitor). All sample panels were then sealed with a HS clear varnish (High Solid). 

The sample panels were measured for colour using the X-Rite MA98 and the BYK-mac I. Both devices each measure at -60°, -30°, -20°, 0°, +30° and +65° in absolute values. These geometries correspond to -15°, 15°, 25°, 45°, 75° and 110° from gloss at an illumination under 45°. Both the reflection colour and the a*b* colour value were assessed, each time against the values of the reference sample. The aforementioned geometries were used in the presentation of the a*b* colour values. The focus of the reflection values assessment was the geometries of as15°- (aspecular 15°) close to the radiance and the as 45° (aspecular 45°) away from the radiance. These geometries are comprised of the illumination at 45° and the observation/measurement at -30° and/or 0°.